U.S. Pat. No. 4,709,385 describes an X-ray diagnostic device of this type in which images of a complete cardiac cycle are read into an image store under ECG control as a mask scene. This means that in fluoroscopic examination mode it is possible to overlay the current video signal with the vessel system's stored video signal which belongs to the same cardiac phase. Such an apparatus is used in scout technology (Road Map).
In cardiology, the beating heart is frequently depicted using X-ray radiation and is stored in electronic form as an image sequence. In this case, the volumes of data which arise may be very large. With an entirely usual resolution of 1024×1024 picture elements and a bit depth of 12 bits—according to a resolution for the brightness in 4096 shades of gray—each image requires at least 1.5 MB. With a frame frequency of up to 30 frames per second in cardiology, or up to 60 frames per second in paediatric cardiology, and a recording period of 10 s for one sequence, a data volume of 450 MB or 900 MB is obtained very quickly. While this volume of data can be regarded as “volatile”, that is to say is buffer-stored only on the X-ray installation itself, it is still possible to handle the volume using current technology. However, as soon as there is the thought of long-term storage and archiving for 20 years or more, logistical problems quickly arise which are dependent on the volume of the cardiological recordings.
Methods for compressing images are known and widespread in image processing. In this case, a distinction needs to be drawn between “lossless” and “lossy” compression. In the case of lossless compression, it is always possible to restore the full, bit-identical image information from the compressed material, whereas in the case of lossy compression only an approximately identical image can be reconstructed. For processing and transporting medical images, the “DICOM” standard is very widespread and accepted by the leading manufacturers of imaging medical products, including in the transmission and storage of cardiological image sequences. The DICOM standard allows not only encoding in the original format and the associated large volumes of data but also lossless compression using the “JPEG lossless” method. This method compresses each individual image in the sequence independently. In typical use, “JPEG lossless” achieves a compression rate of 2:1 to 3:1, that is to say that the compressed data records require less than half of the original space requirement. Lossy compression methods are currently not yet licensed for use on medical image material, for regulatory reasons. It can be expected that particular lossy methods will also be licensed in the near future.
Another approach is to record the fluoroscopic-examination or recording sequences using a conventional video recorder, traditionally through analog recording of a PAL or NTSC converted and undersampled image signal. A reduction in quality is already accepted for this method, and bit-accurate reproduction of the original material is no longer possible. Digital recording with a reproducible result is therefore preferable over analog recording.